1. Field of the Invention
The present invention relates to an electrophotographic image forming technique such as a printer, a copier machine and a facsimile machine, and more particularly, to a liquid development technique which utilizes wet development as a development method and an image forming technique including such a liquid development technique.
2. Description of the Related Art
Such an electrophotographic image forming apparatus has been already commercialized in which exposure means exposes a charged photosensitive member (image carrier) to thereby form an electrostatic latent image on the photosensitive member, developing means makes toner adhere to the photosensitive member, visualizes the electrostatic latent image and accordingly forms a toner image, and the toner image is then transferred onto a transfer medium such as a transfer paper so that a predetermined image is obtained. As a development type used by the developing means, the liquid development is known which uses a liquid developer which is obtained by dispersing charged toner in a carrier liquid. Noting advantages of the liquid development such as that it is possible to obtain a high-resolution image since an average particle diameter of toner is 0.1 through 2 μm, that it is possible to obtain uniform images owing to high liquidity of the solution and other advantages, various types of image forming apparatuses have been proposed.
In an image forming apparatus of the liquid development, when the toner density in a liquid developer changes, the density of a toner image as it is upon visualization of an electrostatic latent image changes. In other words, a change in toner density in the liquid developer is one of major causes of image quality deterioration such as an insufficient optical density and an uneven image. Hence, in order to obtain a stable image, it is necessary to manage the toner density in the liquid developer. In this connection, Japanese Patent Application Laid-Open Gazette No. H11-065300 of 1999 describes an apparatus which detects the viscosity of a liquid developer within a tank which holds the liquid developer which has been collected from developing means, and which adjusts the toner density in the liquid developer which is within the tank in accordance with a result of the detection. This apparatus comprises a liquid developer reservoir which holds the liquid developer which has been collected from a developing belt, separately from a liquid developer storage tank which holds the liquid developer which is to be supplied to the developing belt. A viscometer detects the viscosity of the liquid developer which is within the tank. The viscosity inside the tank is always kept within a tolerable range, as the liquid developer having a high or low density is supplied to the tank when a result of the detection goes outside the tolerable range and thus density-adjusted liquid developer is supplied to the liquid developer reservoir mentioned above from the tank.
U.S. Pat. No. 5,596,396 describes an apparatus which increases the toner density in a liquid developer which is to be supplied to a liquid developer carrier. For simplification of the structure of the apparatus, this apparatus requires to increase the toner density as much as possible in preparation for supplying of the liquid developer to the liquid developer carrier. Further, Japanese Patent Application Laid-Open Gazette No. H10-339990 of 1998 describes an apparatus which turns a liquid developer layer having a high toner density into a thin layer on a liquid developer carrier. In an attempt to improve an image quality, this apparatus requires to create on a developing belt a liquid developer layer which comprises a highly solid area having a high toner density and a surface layer portion having a thin toner density, thereafter remove the surface layer portion and accordingly leave the high-density liquid developer layer as a thin layer.
The apparatus described in Japanese Patent Application Laid-Open Gazette No. 2000-250319 uses a high-viscosity and high-density liquid developer, and requires to remove a carrier liquid from the liquid developer on a photosensitive member after development to thereby improve an image quality.
By the way, when such images are formed continuously having a high image occupation ratio which is a ratio of an image portion to an electrostatic latent image for instance, a large amount of toner adheres on a photosensitive member and a large amount of toner is consumed, while only a small amount of a carrier liquid moves to the photosensitive member from a container which stores a liquid developer. Conversely, when images having a low image occupation ratio are formed successively, since only a small amount of toner adheres on the photosensitive member, more carrier liquid moves to the photosensitive member from the container than during formation of images which have a high image occupation ratio, and much carrier liquid is consequently consumed.
Hence, on those occasions, the necessity of toner density management is particularly high. Yet, in the case of the apparatus described in Japanese Patent Application Laid-Open Gazette No. H11-065300 of 1999, owing to the liquid developer storage tank for collection which is provided separately from liquid developer reservoir which holds the liquid developer which is to be supplied to the developing belt, the apparatus has a big size. Further, since the toner density within the liquid developer storage tank for collection is adjusted and thus density-adjusted liquid developer is supplied to the liquid developer reservoir mentioned above from the tank, the response of thus realized density adjustment to image formation is not good.
Meanwhile, the conventional apparatus described in U.S. Pat. No. 5,596,396 increases the toner density in the liquid developer which is to be supplied to the liquid developer carrier as much as possible for the purpose of simplifying the structure of the apparatus. The conventional apparatus described in Japanese Patent Application Laid-Open Gazette No. H10-339990 of 1998 makes a high-density liquid developer layer thin so as to attain a high image quality. As such, none of these publications is relevant to a technical concept of managing the toner density in a liquid developer.
Further, as described above, the amount of a carrier liquid contained in a liquid developer which moves to a photosensitive member from a container largely changes depending on an image occupation ratio, and this change in turn leads to a change of the toner density in the liquid developer which remains within the container. Despite this, the conventional apparatus described in Japanese Patent Application Laid-Open Gazette No. 2000-250319 merely comprises a structure which removes a constant amount of the carrier liquid off from a photosensitive member, and does not demand to adjust the amount of the carrier liquid to be removed from the photosensitive member in accordance with the amount of the carrier liquid which is on the photosensitive member. Hence, even when thus removed carrier liquid is returned back to the container, it is not possible to suppress a change in toner density in the liquid developer which is within the container.
Further, as described above, the amount of a carrier liquid which moves to a photosensitive member changes greatly depending on the state of a toner image. However, the conventional apparatus described in Japanese Patent Application Laid-Open Gazette No. 2000-250319 merely comprises a structure which removes a constant amount of a carrier liquid off from a photosensitive member, and therefore, cannot respond to a change of the amount of the carrier liquid on the photosensitive member. When the amount of the carrier liquid on the photosensitive member increases for instance therefore, the carrier liquid could be wasted. In addition, a change of the amount of the carrier liquid on the photosensitive member could change a condition of transfer onto a transfer medium and make it difficult to transfer favorably. Hence, one of important control factors for attaining an excellent image quality is to adjust the amount of a carrier liquid contained in a liquid developer on a photosensitive member, namely, the amount of the carrier liquid which is used at the time formation of a toner image.
As another example of a conventional image forming apparatus of the liquid development, Japanese Patent Application Laid-Open Gazette No. H7-209922 of 1995 proposes an apparatus which requires to supply a high-viscosity and high-density liquid developer onto a developer roller and make the liquid developer contact with a photosensitive member to thereby supply the liquid developer onto a latent image surface of the photosensitive member. In this apparatus, as such a bias is applied which will cause migration of charged toner toward the developer roller at the time of contacting of the liquid developer on the developer roller with the photosensitive member for instance, it is possible to prevent the charged toner from moving toward the photosensitive member. However, since a carrier liquid will inevitably adhere to a photosensitive member after contacting the photosensitive member, it is not possible to prevent the carrier liquid from moving toward the photosensitive member. In the conventional apparatus described in Japanese Patent Application Laid-Open Gazette No. H7-209922, too, since a liquid developer on a developer roller is always in contact with a photosensitive member, a carrier liquid always moves from the developer roller toward the photosensitive member. As a result, when the liquid developer is not in demand because of the state of toner image formation, the carrier liquid is wasted.